1. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a method of predicting received signal strength in a discontinuous radio transmission system.
2. Background Art
The present invention is directed particularly to discontinuous radio transmission systems. One such system in common use is known as GSM. In 1982, a study group called the Groupe Spécial Mobile (GSM) was formed to study and develop a pan-European public land mobile system. In 1990, phase I of the GSM specifications were published. Commercial service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries. Although standardized in Europe, GSM is not only a European standard. Over 200 GSM networks (including DCS1800 and PCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3 million subscribers worldwide, which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.
The GSM network can be divided into three broad parts. The mobile station is carried by the subscriber. The base station subsystem controls the radio link with the mobile station. The network subsystem, the main part of which is the mobile services switching center, performs the switching of calls between the mobile users, and between mobile and fixed network users.
Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms. Eight burst periods are grouped into a TDMA frame, which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.
Minimizing co-channel interference in a cellular system allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a mode of operation that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation, by turning the transmitter off during silence periods. Reducing the transmission time through DTX reduces co-channel interference. An added benefit of DTX is that power is conserved at the mobile unit.
Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobile station needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into sleep mode, when almost no power is used.
A common implementation of a mobile station receiver has an analog section that amplifies the received signal such that it can be quantized with minimal quantization or saturation noise. The amount of amplification required is inversely proportional to the received signal power.
In a DTX system the mobile station does not know the signal strength of the next received signal. The signal strength or received power must be predicted to correctly set the level of receiver amplification. Using the last received signal power to set receiver amplification could cause excessive saturation or quantization noise.
At the assigned 900 MHz frequency band, GSM radio waves bounce off objects such as buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. The interference caused by these reflected signals is known as multipath fading.
One of the variables affecting received power is the multipath fading that occurs in the GSM radio frequency channel. Multipath fading follows a Rayleigh distribution, if only locally reflected waves are taken into account, and therefore multipath fading is frequently called Rayleigh fading.
Rayleigh fading has the property of having larger attenuation, but for a shorter duration, than gain relative to its mean. The previously received signal may have been subject to large attenuation (known as a deep fade) because of Rayleigh fading. Therefore, this power level is not a good estimate of the next signal to be received.
What is needed is a simple and reliable method to predict the power of a next received signal. The method should be applicable to a discontinuous transmission system and mitigate the previously discussed errors.